Process for producing a tough steel for low temperatures



3,153,565 PRGCESS FQR PRODUUNG A TUUGH STEEL FOR LQW TEMPERATURESKamelrichi Wada, Yawata, Japan, assignor to Yawata Iron & Steel Co.,Ltd, Tokyo, Japan, a corporation of Japan No Drawing. Filed June 19,1962, Ser. No. 203,444 Claims priority, application Japan, June 22,1961, 3/22,237 4 Claims. (Cl. 148-143) This invention relates to theprocess for producing steel having high weldability and toughness at lowtemperatures. to a process for producing tough steel for lowtemeperature service which contains no costly nickel and is easy toproduce, simple to heat-treat and low in cost.

Generally, a steel has a transition temperature which is determined byits constituency and will absorb high energy at the time of breaking onthe higher side of the transition temperature and'low energy on the lowtemperature side. Therefore, it is considered necessary that a steel tobe used at low temperatures should have a transition temperature lowenough for the conditions which it is to be used under and further,should be tough.

A steel which has been extensively used for this purpose contains morethan 2.5% nickel in order to keep the transition temperature low.However, when the alloying element nickel is added, the steel becomescostly and, further, special consideration is required for refining,ingoting, heating and rolling in the production of the steel andtherefore the production cost is high.

The object of the present invention is to eliminate the aforementioneddefects and to provide low-temperature tough steel which is easy toheat-treat and is low in cost.

A steel according to the present invention .can easily be made bymelting in an open-hearth furnace, electric furnace or pure oxygenconverter. Its chemical composition is approximately less than 0.14%'carbon, less than 0.5% silicon, 1.0 to 1.5% manganese, 0.01 to 0.04%aluminum and 0.008 to 0.020% nitrogen, less than 0.20%- of one or moreof the elements niobium, tantalum and vanadium if required with theremainder being iron and unavoidable impurities. Small amounts ofsomespecial element may be added when required. Steel having suchacomposition is produced by first melting, then rolling or forging, thenheating to a temperature above 880 C., then quenching, further reheatingto 600 to 700 C. and then cooling by air or the like.

Thus, the first of the fundamental features of the present invention isthat when the aluminum and nitrogen are in a solid solution andsupercooledin the steel they are deposited as aluminum nitride byheating from 600 to 700 C., which is below the A transformation point,and then cooling so that the toughness at low temperatures may beimproved. As already. known, it is possible to improve the toughness bydepositing aluminum nitride near the A transformation point. However,the method of increasing the toughness by depositing aluminum nitridebelow the A, transformation point, as in the present invention, is farmore effective than any known conventional method. The present inventionis the first to discover and apply such fact. When the above mentionedheating temperature is below 600 C., the deposition of aluminum nitridewill be volumetrically insufficient. When it is above 700 C., thestructure will become austenite and the structure and deposited statewill be nonuniform and undesirable.

United States Patent More particularly the present invention relates"ice The second of the features is to produce a steel having a fineferrite structure. That is, said aluminum nitride will be highlyeffective when, it is deposited in ferrite. Therefore, the steel israpidly cooled from above 880 C. so that the steel will not haveso-called hardened structure such as martensite or bainite but rather,should have a ferrite structure and that, before treating to deposit thealuminum nitride both the nitrogen and aluminum should be in solidsolution in the steel.

In order that the aluminum and nitrogen are well liberated, the thermalcondition before the depositing treatment must be restricted. Theheating temperature above 880 C. in the heat-treatment of the steelaccording to the present invention is determined therefore. It ispreferable that the heating time is as short as possible. However, asdescribed later, the heating temperature must be kept in such range aswill not make the crystal grain size coarse. The heating temperaturebefore cooling is recommended to be about 880 to 950 C.

The cooling rate should be so rapid that no aluminum nitride will bedeposited during cooling and must be controlled so that fine ferritewill appear and no hardened structure is made. A cooling rate of about 5to 50 C./sec. is recommended. Further, in order to prevent aluminumnitride from being deposited, the COO1-' below the A transformationpoint. The carbon content.

is below 0.14% by weight so that a ferrite structure is obtained at thetime of rapid cooling.

The silicon content is below 0.5%.by weight which is the normalrequirement in steel making. The manganese content is as high as ispossible and is about 1.0 to

1.5% by weight so that, as already known, the toughness at lowtemperatures is increased and the weldability is improved.

Further, if necessary, there may be added a small amount, e.g., lessthan 0.2% by weight of one or more of the elements niobium, tantalum andvanadium each of which will form a fine carbide, stable even at hightemperatures, and will reduce the hardenability, accelerate theproduction of fine ferrite and, at the same time, improve theweldability of the'steel.

EXAMPLE A'steel containing the ingredients shown in Table 1 was producedby heating in an electric furnace to a temperature 'of 930 C. in 20minutes. The steel was then rapidly cooled at a rate of 15 C./sec., thenheated to a temperature of 625 C. for 30 minutes and was then cooled inair.

Table 1.-Examples of Chemical Compositions in Percent Sample 0 Si Mn AlN Nb I V Ta signs 3 3 Table 2.--Examples of Mechanical Properties [Asheated to 930 C. for 20 minutes, rapidly cooled, then heated to 625 C.[or 30 minutes and cooled in air] Press notch Charpy Sample YieldingTensile Elongation broken signs point in strength in percent] surfacelrgJnun. kg/mm. 200 mm. transition temperature in C.

observed. However, it is clearly seen that the sample which was rapidlycooled had better values than the rolled sample which is explained bythe fact that there is a difference in the ferrite grain size betweenthe two samples which is caused by the heating of the one sample to a Table 3.-Examples of Deposition of Aluminum Nitride in Weight Percent byHeat-Treatment Table 4.Examples of the Relation Between the Content ofNitrogen as Aluminum Nitride and Press Notch Charpy TransitionTemperature [With the composition of Sample D in Table 1] (a) In thetable below the sample as rolled was heated to 625 C. for 30 minutes andwas cooled in air. Ferrite-grain size No. 79 (A.S.T.M.).

Content of nitrogen in weight percent as aluminum nitride .1 0. 0029 0.0079 0. 0144 Press notch Charpy broken surface transition temperature inC 18 -21 -38 -42 (b) In the table below the steel was heated to 930 0.for minutes,

rapidly cooled, then heated to 625 C. for minutes and cooled in air.Ferrite-grain size No. 911 (A.S. T.M.).

Content of nitrogen in weight percent as aluminum nitride 0 0031 0.00830. 0152 Press notch Charpy fracture transition temperature in C 28 -3461 .58

utes and then cooled in air, sample 3 is the same as sample 1 except itwas heated to 930 for 20 minutes and then rapidly cooled and sample 4 isthe same as sample 3 except it was heated to 625 C. for 30 minutes andthen cooled in air. In (a) and (b) in Table 4 there is shown therelationship between the amount of aluminum nitride and the press notchCharpy broken surface tran sition temperature on the rolled sample andthe sample heated to 930 C. for-20 minutes and rapidly cooled, both ofwhich were heated to 625 C. for 30 minutes and then cooled in air forthe depositing treatment. In both the rolled .sample and the rapidlycooled sample, after the depositing treatment, aluminum nitride wasgreatly deposited-and an increase in low-temperature toughness wasNitrogen as aluminum nitride Aoid- Sample 4 (as heated soluble TotalSample 2 (as heated Sample 3 (as heated to 030 C. for 20 aluminumnitrogen Samplel to 625 C. for 30 to 030 C. for 20 minutes, rapidly (asrolled minutes and cooled minutes and rapidly eooled,then heated only)in air) cooled) to 62 C. for 30 7 minutes and cooled inair) two below600 C. whereby there is produced a fine ferrite structure in said steeland the liberated aluminum and nitrogen are in solid solution; reheatingthe cooled steel to a temperature of about 600 to 700 C. and cooling thesteel whereby the aluminum and nitrogen are deposited in said steel asaluminum nitride.

2. A process for producing a tough steel for low temperature servicecomprising heating a low carbon steel containing 0.01 to 0.04 weightpercent aluminum, 0.008 to 0.02 weight percent nitrogen, less than 0.50%silicon and less than 0.14 weight percent carbon to a temperature ofabout 88010 950 C, quiclily cooling said steel at a cooling rate of from5 to 50 degrees centigrade per second to a temperature below 600 C.wherehythere is produced a fine ferrite structure in said. steel and theliberated aluminum and nitrogen are insolid solution; reheating thecooled steel to a temperature of about 600 to 700 C. and cooling thesteel to room temperature whereby the aluminum and nitrogen aredeposited in said steel as aluminum nitride.

3. A process for producing a tough steel for low temperature servicecomprising heating a steel containing 1.0 to 1.5 weight percentmanganese, 0.01 to 0.04 weight percent aluminum, 0.008 to 020 Weightpercent nitrogen, less than 0.14 weight percent carbon, less than 0.50weight percent silicon with the remainder being iron and incidentalimpurities to a temperature of about 880 to 950 C. and quickly coolingsaid steel at a cooling rate of from 5 to 50 degrees centigrade persecond to a temperature below 600 C. whereby there is produced a fineferrite structure in said steel and the liberated aluminum and nitrogenare in solid solution; reheating the cooled steel to a temperature ofabout 600 to 700 C. and cooling the steel to room temperature wherebythe aluminum and'ni-,

trogen are deposited in said steel as aluminum nitride.

4. A process for producing a tough steel for low temperature servicecomprising heating a steel containing 1.0 to 1.5 weight percentmanganese, 0.01 to 0.04 weight percent aluminum, 0008 to 0.20 weightpercent nitrogen, less than 0.14 weight percent carbon, less than 0.50weight percent silicon and at least one alloying element selected fromthe group consisting of niobium, tantalum and vanadium, the total amountof said alloying elements T5 6 being less than 0.20 weight percent withthe remainder References Cited by the Examiner being iron and incidentalimpurities to a temperature of UNITED TATE PATENTS about 880 to 950 C.and quickly cooling said steel at a S S cooling rateof from 5 to 50degrees centigrade per second 3,028,270 4/62 Monta 148 143 to atemperature below 600 C. whereby there is pro- 5 FOREIGN PATENTS duced afine ferrite structure in said steel and the liberated 786993 11/57Great Britain aluminum and nitrogen are in solid solution; reheating the2/59 Great Britain cooled steel to a temperature 'of about 600 to 700 C.and cooling the steel to room temperature whereby the DAVID RECK, 'yExamineraluminum and nitrogen are deposited in said steel as 10 RAY K.WINDH AM Examiner aluminum nitride.

1. A PROCESS FOR PRODUCING A TOUGH STEEL FOR LOW TEMPERATURE SERVICECOMPRISING HEATING A LOW CARBON STEEL CONTAINING 0.01 TO 0.04 WEIGHTPERCENT ALUMINUM, 0.008 TO 0.02 WEIGHT PERCENT NITROGEN AND LESS THAN0.14 WEIGHT PERCENT CARBON TO A TEMPERATURE ABOVE THE A3 TRANSFORMATIONPOINT, QUICKLY COOLING SAID STEEL AT A COOLING RATE OF FROM 5 TO 50DEGREES CENTIGRADE PER SECOND TO A TEMPERATURE BELOW 600*C. WHEREBYTHERE IS PRODUCED A FINE FERRITE STRUCTURE IN SAID STEEL AND THELIBERATED ALUMINUM AND NITROGEN ARE IN SOLID SOLUTION; REHEATING THECOOLED STEEL TO A TEMPERATURE OF ABOUT 600 TO 700*C. AND COOLNG THESTEEL WHEREBY THE ALUMINUM AND NITROGEN ARE DEPOSITED IN SAID STEEL ASALUMINUM NITRIDE.